Resistivity logging, which measures the electrical resistivity of formations surrounding an earth borehole, is a commonly used technique of formation evaluation. For example, porous formations having high resistivity generally indicate the presence of hydrocarbons, while porous formations having low resistivity are generally water saturated. In so-called "wireline" well logging, wherein measurements are taken in a well bore (with the drill string removed) by lowering a logging device in the well bore on a wireline cable and taking measurements with the device as the cable is withdrawn, there are several techniques of resistivity logging which use elements such as electrodes or coils. Various arrangements of electrodes, on the logging device and at the earth's surface, have been utilized to measure electrical currents and/or potentials from which formation resistivity can be derived. For example, button electrodes have been employed on a pad which is urged against the borehole wall. These electrodes have been used to obtain azimuthal resistivity measurements, and focusing techniques have been employed to obtain resistivity measurements that have substantial lateral extent into the formations and provide relatively high vertical resolution resistivity information.
Various techniques for measuring resistivity while drilling have also been utilized or proposed. Techniques employed in wireline logging may or may not be adaptable for use in logging-while-drilling equipment. The borehole presents a difficult environment, even for wireline logging, but the environment near the well bottom during drilling is particularly hostile to measuring equipment. For logging-while-drilling applications, the measuring devices are housed in heavy steel drill collars, the mechanical integrity of which cannot be compromised. Measurement approaches which require a substantial surface area of electrically insulating material on the surface of a drill collar housing are considered impractical, since the insulating material will likely be damaged or destroyed. This is particularly true for measuring structures that would attempt to attain intimate contact with the newly drilled borehole wall as the drill string continues its rotation and penetration, with the attendant abrasion and other stresses.
One resistivity measuring approach is to utilize a plurality of toroidal coil antennas, spaced apart, that are mounted in insulating media around a drill collar or recessed regions thereof. A transmitting antenna of this nature radiates electromagnetic energy having a dominant transverse magnetic component, and can use the electrically conductive body of the drill collar to good advantage, as described next.
In U.S. Pat. No. 3,408,561 there is disclosed a logging-while-drilling system wherein a receiving toroidal coil is mounted in a recess on a drill collar near the drill bit and a transmitting toroidal coil is mounted on the drill collar above the receiver coil. The drill collar serves as part of a one-turn "secondary winding" for the toroidal antennas, the remainder of such "secondary winding" including a current return path through the mud and formations. The voltage induced in the receiver toroidal coil provides an indication of the resistivity of formations around the drill bit. U.S. Pat. No. 3,305,771 utilizes a similar principal, but employs a pair of spaced-apart transmitting toroidal coils and a pair of spaced-apart receiving toroidal coils between the transmitting toroidal coils.
As generally described in the prior art, a transmitter toroidal coil mounted on a drill collar induces current in the drill collar which can be envisioned as leaving the drill collar, entering the formations below the transmitter coil, and returning to the drill string above the transmitter coil. Since the drill collar below the transmitter coil is substantially an equipotential surface, a portion of the current measured by a lower receiver toroidal coil mounted near the drill bit tends to be laterally focused. This can provide a "lateral" resistivity measurement of formations adjacent the drill collar. Also, a portion of current leaving the drill stem below the receiver coil provides a "bit resistivity" measurement; that is, a measurement of the resistivity of the formations instantaneously being cut by the bit. [See, for example, the above-identified U.S. Pat. Nos. 3,408,561 and U.S. Pat. No. 3,305,771, and publications entitled "A New Resistivity Tool For Measurement While Drilling", SPWLA Twenty-Sixth Annual Logging Symposium (1985) and "Determining The Invasion Near The Bit With The MWD Toroid Sonde", SPWLA Twenty-Seventh Annual Logging Symposiuan (1986).]Thus, the prior art indicates that a measurement-while-drilling logging device using toroidal coil transmitting and receiving antennas can be employed to obtain lateral resistivity measurements and/or bit resistivity measurements.
Reference can also be made to the following which relate to measurement-while-drilling using electrodes and other transducers: U.S. Pat. No. 4,786,874, U.S. Pat. No. 5,017,778, and U.S. Pat. No. 5,130,950.
Resistivity measurements obtained using transmitting and receiving toroidal coils on a conductive metal body are useful, particularly in logging-while-drilling applications, but it is desirable to obtain measurements which can provide further information concerning the downhole formations; for example, lateral resistivity information having improved vertical resolution, azimuthal resistivity information, and multiple depths of investigation for such resistivity information.
In copending U.S. Pat. No. 5,235,285; assigned to the same assignee as the present application, there is disclosed an apparatus utilizing a toroidal coil antenna mounted, in an insulating medium, on a drill collar to induce a current which travels in a path that includes the drill collar and earth formations around the drill collar. [See also U.S. Pat. No. 5,200,705 assigned to the same assignee as the present application.] As was generally known in the art, one or more toroidal coil receiving antennas can be mounted, in an insulating medium, on the drill collar to obtain the types of measurements described above. The apparatus of the referenced copending Application expands on the toroid-to-toroid type of measurement to obtain further useful information about the downhole formations. In one form thereof at least one electrode is provided on the drill collar and is utilized to detect currents transmitted by the transmitter toroidal coil which return via the formations to the electrode(s) laterally; that is, approximately normal to the axis of the drill collar. The electrodes preferably have a relatively small vertical extent, and the measurements taken with these electrodes are useful in obtaining formation resistivity with relatively high vertical resolution, as well as relatively high depth of investigation for the resolution provided. The electrodes can also provide azimuthal resistivity information. Thus, resistivity logging measurements are obtained that can supplement or replace resistivity measurements obtained with toroidal coil receiving antenna(s). The electrode(s) can be mounted in a drill collar or on a stabilizer blade attached to or integral with the drill collar. In embodiments thereof, button-type electrodes are utilized, as well as ring-type electrodes. The one or more receiving ring electrode or button electrode can be electrically isolated from the main metal body of the drill collar, using rubber or other insulating material, and the electrical potential kept at the same value as the surrounding metal. The current leaving the electrode can be measured, and the measurement ideally would determine the resistivity of formations in the region immediately surrounding the electrode. However, under certain conditions the measurement may not be accurately representative of the resistivity of the region immediately surrounding the electrode, and it is among the objects of the invention to provide improvement in accuracy of logging devices under such conditions.